Application of paleoporosity and bitumen saturation concepts to tight-gas accumulations containing solid bitumen

Abstract

Solid bitumen significantly influences reservoir quality in many unconventional tight and shale hydrocarbon accumulations. In the siltstone-dominated Montney Formation of northeast British Columbia and northwest Alberta organic matter is mostly in the form of solid bitumen and represents an original pore-filling oil phase that was later thermally cracked. Here, we show that the influence of solid bitumen on reservoir quality can be further understood by applying the concepts of paleoporosity and bitumen saturation. Paleoporosity is defined as the sum of present-day porosity and total organic carbon (TOC) volume, and conceptualized as the porosity available at the time of oil charging. Bitumen saturation is defined as TOC volume divided by paleoporosity, and conceptualized as the fraction of paleoporosity filled with oil/bitumen. We introduce the paleoporosity-bitumen saturation cross-plot and use it, together with organic petrography and scanning electron microscopy observations, to investigate how different combinations of paleoporosity and bitumen saturation impact rock properties. Solid bitumen in the Montney Formation influences pore throat size, porosity, permeability and wettability. Understanding such key rock properties benefits from recognizing paleoporosity and bitumen saturation as inherent rock attributes with wide-ranging combinations. The concepts of paleoporosity and bitumen saturation elucidated here can likely be applied to other tight hydrocarbon accumulations that contain organic matter dominantly in the form of solid bitumen.